It has long been recognized that success in the market place, particularly with respect to disposable diapers incontinence garments or pads and the like, depends substantially on the functional efficiency, appearance, and price of the products being sold.
In general, such products must have a fluid-absorbent core, usually comprising one or more layers of absorbent material such as wood pulp, rayon, gauze, tissue or the like, and, in some cases, synthetic hydrophilic material such as a polyurethane foam.
To protect clothing, and surrounding areas from being stained or wetted by fluids retained in such pad or core, it is also generally backed by a fluid-impervious barrier sheet.
Such fluid-absorbent pad or core generally also has a facing of coverstock material, which covers at least the body-contacting surface of the product.
The fluid-absorbing product is generally packaged in the form of a loosely bonded pad which may have a rectangular or somewhat oval shape.
The functional purpose of such coverstock is two-fold, namely (1) to promote durability by helping to contain the loosely packed pad or core of absorbent material and (2) to insulate the wearer from continuous direct contact with moisture already retained in the absorbent pad or core. Such facing or coverstock must be pervious to fluids on the side of the product that is placed against the body, so as to promote the direct and immediate transfer of each fluid application or insult into the absorbent core; the coverstock itself must also be essentially nonabsorbent to fluid and remain dry. It is particularly important, in this regard, to minimize potential lateral migration of fluid along the coverstock surface, even after repeated insults, and for the coverstock to continue to feel dry, smooth and soft to the touch.
Certain additional characteristics are also sometimes desired, such as visual opacity, specific coloring or luster on the outer surface, and fabric acceptance of external designs.
It is now recognized that the comfort factor (i.e. dryness, softness, resistance to rewet etc.) of diapers, pads, and the like can be substantially improved by increasing thickness or caliper of the nonwoven coversheet, to obtain increased softness while retaining a well-defined separation between the wearer's skin and the fluid-retaining core.
Such increased caliper can be conveniently obtained by increasing the basis weight of the coverstock or by decreasing its density (i.e. increase loft). Of the two, the later is generally favored, but at a sacrifice in durability and strength.
A number of approaches have been offered in the art for obtaining bulkier coverstock. For example, U.S. Pat. No. 4,041,951 teaches embossing a nonwoven topsheet and U.S. Pat. No. 4,391,869 proposes limiting the amount of aqueous binder applied in a vacuum-bonding operation with airlaid nonwovens. More recently, thru-air-bonded bicomponent fiber structures have been considered. Added detail concerning general use of thru-air bonding techniques is found in an article entitled "Multi-layer Nonwovens for Coverstock, Medical, and other End Uses" by J. Pirkkanen in the November 1987 issue of "Nonwovens World".
This reference utilizes multilayer nonwoven fabrics having a basis weight of about 30 grams per square meter.
U.S. Pat. No. 4,548,856 and U.K. patent application GB 2,127,865A are also of interest in disclosing thru-air bonding procedures involving multibelt systems which form patterned nonwoven fabric.
Also of interest is U.S. Pat. No. 4,652,484 (assigned to Kao) which proposes forming a diaper liner having a layered structure, in which the first layer is comprised of 1-3 denier straight bicomponent fiber and the second layer of sterically buckled (three-dimensional crimp) 1.5 to 6 denier bicomponent fiber. This patent proposes achieving optimum thickness and softness through use of sterically-buckled bicomponent fiber.
Some degree of improvement in mechanical strength is obtained in U.S. Pat. No. 4,761,322, (Kimberly-Clark Corporation), through use of a top web layer of nonwoven laminate, which is densely calender bonded to a bottom web layer of substantially lower bonding density.
To date, however, the art has not been able to provide nonwovens with the desired high loft while retaining flow through and sufficient dimensional stability and durability to meet market needs.
It is an object of the present invention to obtain a high loft soft nonwoven material suitable as a diaper coverstock or the like without sacrificing durability, or strength, and while retaining required flow-through and rewet properties.